barisan aritmatika sn=77,b=2 dan n=7 suku pertama barisan tsb adalah

sn = n/2 (2a+(n-1)b)

77 = 7/2 ( 2a+(7-1)2)
77= 7/2 (2a+6.2)
77= 7/2 (2a+12)
77.2 = 7(2a+12)
154 = 14a+84
154-84 = 14a
70= 14a
a= 70/14
a= 5

sn=77 b=2 n=7


S7=77

S7=77/2
[tex]s7 = \frac{7}{2} (a + (7 - 1) \times 2 \\ \frac{7}{2} (a + 12) \\77 = 7\ \ \times (a + 6) \\ 77 = 7a + 42 \\ \ \\ 7a = 77 - 42 \\ 7a = 35 \\ a = \frac{35}{7} \\ a = 5[/tex]